Method of growing single crystals of compounds

ABSTRACT

In growing a single crystal of a compound or an alloy composed of a plurality of compounds, having a volatile element, from a melt of the compound in a sealed ampoule, a quantity of the volatile element is provided to maintain an atmosphere of the volatile element over the melt and the temperature of the reservoir is slowly cycled around a given temperature, thereby cycling the vapor pressure of the volatile element of single crystal in the product.

[ Apr. 15, 1975 United States Patent [191 Berkman et al.

[ METHOD OF GROWING SINGLE 3,366,454 1/1968 Folberth Y TAL F OMPOUNDS3,520,810 7/1970 Plaskett et a1.

CR S S 0 C 3,729,348 4/1973 Saul [75] Inventors: Samuel Berkman, FlorhamPark,

N.J.; Philip Michael Britt, Manheim, Pa.

Primary ExaminerA. Louis Monacell Assistant Examiner-S. J. Emery [73]Assignee: RCA Corporation, Princeton, NJ. Attorney, Agent, or FirmGlennH. Bruestle; William Filed: July 13, 1973 S. Hill [57] ABSTRACT Ingrowing a single crystal of a compound or an alloy composed of aplurality of compounds, having a vola- [21] Appl. No.: 378,818

U.S. 23/301 SP; 23/305; 423/111;

P0 So 37 71 U 0 2 B [51] Int Cl tile element, from a melt of thecompound in a sealed [58] Fie'ld 273 SP ampoule, a quantity of thevolatile element is provided to maintain an atmosphere of the volatileelement 423/111 over the melt and the temperature of the reservoir isslowly cycled around a given temperature, thereby cycling the vaporpressure of the volatile element of single crystal in the product.

S T N m n we m e y e D E u N U M U MP. I240C in wmoow TEMP. SLOPE 20mmCOLD ZONE CONTROL TEMP 630C 3,260,573 7/1966 Ziegler...................3,318,669 5/1967 Folberth...............................

2 2 l m 4, 1 7 ,1? n1; 11/ l 1/ 2 v t 0 8 3 I ll l l 2 4 I 6 l I 4 l L 11 v f 1 Au 4 4 H I 8 I 2 D 00, F. B H/ E T A I. 6 6 1 I 4 1 2 z 1/ C 4 6v 2 1 A H 6 I3 I a 3 0 4 4 07 /9 I 17 5 1 r CARRIAGE BACKGROUND OF THEINYEN'IION Gallium arsenide single crystals are used for makinginjection lasers (among other things). This application requires thatthe crystals be of high quality with a low dislocation density.

Although single crystals of gallium arsenide can be grown by any one ofseveral methods. a method of particular interest is one in which a seedcrystal is placed in Contact with a melt of gallium arsenide in a boatwithin a horizontal Bridgman furnace. The boat and a quantity of arsenicare contained within a sealed ampoule. The arsenic is maintained at atemperature cooler than that of the boat containing the seed and themelt but warm enough to maintain arsenic vapor at a pressure of l to 1.5atmospheres over the melt. A crystalis grown by moving the boat and aheat zone with respect to each other so that the melt is progressivelyfrozen beginning at the seed end and progressing toward the oppositeend.

In the past. great care was exercised to try to keep the arsenic vaporpressure as constant as possible within the ampoule as the crystal wasgrown. This was accomplished by keeping the temperature of the arsenicas constant as possible. A sensing thermocouple was disposed adjacentthe arsenic and the thermocouple was used to control the currentsupplied to the zone heater. However. it was found that. despite thegreat care taken to maintain constant arsenic vapor pressure. singlecrystal yield has averaged about 50%.

The present invention is based on the discovery that single crystalyield of a compound such as gallium arsenide that contains a relativelyvolatile component. can be greatly increased by slowly cycling thetemperature of the additional quantity of volatile element around a meantemperature. thereby also cycling the volatile element vapor pressurewithin the ampoule.

THE DRAWING FIG. I is a schematic section view of apparatus that can beused to carry out the method of the present invention. as it appearsduring a preliminary stage of the method. together with a typicaltemperature furnace profile for growing a crystal of gallium arsenide.and

FIG. 2 is a somewhat enlarged cross-section view of part of theapparatus shown in FIG. I. as a crystal is being grown.

DESCRIPTION OF PREFERRED EMBODIMENT Apparatus. as shown in FIG. 1., maybe used to carry out the improved crystal growing method ofthe presentinvention. The apparatus may comprise an electrically heated furnace 2having controlled heating elements for providing a relatively cold zone4. a zone 6 of intermediate heat and a hot zone 8. The heated zones aresurrounded with firebrick insulation 10. The furnace has a centralcylindrical chamber 12 extending horizontally from end-to-end.

For convenience in observing the progress of the crystal growth. anobservation window 14 extending through the firebrick insulation and theheating coil of the hot zone 8. is provided.

The entire furnace 2 is movably carried on a lathe bed carriage 16 whichhas means (not shown) for moving the furnace in a steady motion at acontrolled rate along a limited path. The furnace of this example has alength of 36 inches.

Within the central chamber 12 of the furnace 2 is suspended a mullitetube 18 mounted on end supports 20 and 22 which are outside the furnace.The tube 18 has an outer diameter which is slightly less than the innerdiameter ofthe furnace chamber l2 so that the tube 18 and the furnace 2can move freely with respect to each other.

Within the mullite tube 18 is a quartz ampoule 24. The mouth of theampoule is closed with a glass seal 26 after the ampoule is loaded aswill be described later.

Within the ampoule 24 is a quartz. boat 28. To grow a crystal of galliumarsenide. for example. the boat 28 is loaded by placing a seed crystal30 of gallium arsenide properly oriented at one end of the boat. Therest of the boat 28 is filled with a mass 32 of polycrystalline galliumarsenide. A quantity of arsenic 34 is placed on the bottom of theampoule 24 between the boat 28 and the cold end of the ampoule (seal26). After the ampoule 24 is loaded. the ampoule is evacuated to apressure of about IO' Torr and closed by means of the glass seal 26.Alternatively. the gallium arsenide can be prepared within the boat byreacting gallium metal and arsenic vapor.

In order to aid in accurately controlling the temperature of the coldzone 4 of the furnace. a thermocouple 36 is provided adjacent the glassseal 26 within the ampoule 24. The thermocouple 36 has lead wires 38 and40 which are connected to a programmer and current control device (notshown). Another thermocouple 42 is provided within the hot zone 8 to aidin accurately controlling the temperature at the interface between theseed crystal 30 and the liquid melt 32' (FIG. 2) which results frommelting the polycrystalline mass 32. The thermocouple 42 has lead wires44 and 46 connected to a current control device (not shown).

When a crystal is to be grown. the ampoule 24 is disposed within themullite tube 18 so that the boat 28 is within the hot zone 8 of thefurnace 2 and the peak temperature portion of the hot zone extends onlyto the interface between the seed crystal 30 and the melt 32'. Theobservation window 14 aids the operator in accurately positioning theampoule 24 and boat 28. The opposite end of the ampoule containing thearsenic supply 34 must be well within the cold zone 4.

After the ampoule 24 is properly positioned. current is turned on to thethree zones 4, 6 and 8 of the furnace 2. The furnace has the temperatureprofile shown in FIG. 1 of the drawing.

When the furnace 2 has become fully heated. the polycrystalline mass 32becomes molten. Also some of the arsenic 34 is vaporized and an arsenicatmosphere becomes established throughout the ampoule 24 including thespace above the boat 28. Preferably. the arsenic vapor is at a pressureabout I to L5 times normal atmospheric pressure (at sea level). Some ofthe arsenic supply 34 condenses on the cold wall of the seal 26 to forma deposit 34'.

To grow a crystal. the furnace 2 is moved on the lathe carriage 16 sothat the hot zone 8 very slowly (e.g.. one-fourth inch per hour) movesaway from the seed crystal 30. Successive portions of the melt 32'solidify to form a single crystal 48 attached to the seed 30.Ordinarily. in a process of this type. usable single crystal yieldaverages only about 50%.

In the improved method of the present invention. the temperature of thecold zone 4 is very slowly cycled in a sinusoidal manner within a rangebetween l to +l C and 5 to +5 C with respect to the normal meansublimation temperature of the arsenic. The cycle period is about It) to30 minutes. The temperature cycling causes a corresponding cycling ofthe arsenic vapor pressure. 1

When crystals are grown by this method. usable single crystal yieldapproaches 95 108% of lowdislocation crystal.

Although the method has been illustrated in growing crystals of galliumarsenide. it applies equally well to growing crystals of other lll-\compounds in which one of the elements is so volatile that an atmosphereof the volatile element must be maintained over the melt which is beingsolidified so that the melt will not become non-stoichmetric due to lossof some of one of the constituents. Examples of other suitable compoundsare indium arsenide and gallium phosphide. The method also is notlimited to lll-V compounds.

The method is also applicable to other crystal growing methods. forexample. the vertical Bridgman method. horizontal gradient freezemethod. vertical gradient freeze method. and traveling solvent method.

We claim:

1. In a method of growing a single crystal of a lll-V compound. oneelement of which is relatively volatile with respect to another elementof the compound. by maintaining a seed crystal of the compound incontact with a melt of the compound within a sealed chamber disposedwithin a furnace and progressively moving a hot zone relative to saidseed and said melt. away from said seed. and in which a quantity of saidrelatively volatile element is maintained within said sealed chamberadjacent said melt but at a lower mean temperature than that of saidmelt such that a part of said relatively volatile element becomesvaporized and such that an atmosphere of said relatively volatileelement is maintained over said melt. the step of continuously cyclingthe temperature of said quantity of said volatile element after it hasbecome vaporized. around said mean temperature within a range between lto +lC and -5 to +5C, thereby cycling the pressure of said atmosphere.the period of said cycling being about It) to 30 minutes.

2. A method according to claim 1 in which said compound is galliumarsenide.

3. A method according to claim 1 in which the single crystal is grown ina horizontal Bridgman type furnace. =l

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATEM' NO. 3,s77,sss

DATED April 15, 1975 mvEMORm Samuel Berkman/Philip Michael Britt i! iscertified that error appears in the above-identified patent and thatsaid Letters Patent are here}; corrected as shown below:

In the "Abstract" change the last two lines to read:

cling the vapor pressure of the volatile element within the ampoule.This results in a higher percentage of single crystal in the product.

In the specification:

Col. 3 line 10, change "108%" to 100% gigncd and Scaled this A nest:

RUTH C. MASON C. MARSHALL DANN Alresrr'ng Officer Commissioner ofParents and Trademarks

1. IN A METHOD OF GROWING A SINGLE CRYSTAL OF A III-V COMPOUND, ONE ELEMENT OF WHICH IS RELATIVELY VOLATILE WITH RESPECT TO ANOTHER ELEMENT OF THE COMPOUND, BY MAINTAINING A SEED CRYSTAL OF THE COMPOUND IN CONTACT WITH A MELT OF THE COMPOUND WITHIN A SEALED CHAMBER DISPOSED WITHIN A FURNACE AND PROGRESSIVELY MOVING A HOT ZONE RELATIVE TO SAID SEED AND SAID MELT, AWAY FROM SAID SEED. AND IN WHICH A QUANTITY OF SAID RELATIVELY VOLATILE ELEMENT IS MAINTAINED WITHIN SAID SEALED CHAMBER ADJACENT SAID MELT BUT AT A LOWER MEAN TEMPERATURE THAN THAT OF SAID MELT SUCH THAT A PART OF SAID RELATIVELY VOLATILE ELEMENT BECOMES VAPORIZED AND SUCH THAT AN ATMOSPHERE OF SAID RELATIVELY VOLATILE ELEMENT IS MAINTAINED OVER SAID MELT, THE STEP OF CONTINUOUSLY CYCLING THE TEMPERATURE OF SAID QUANTITY OF SAID VOLATILE ELEMENT AFTER IT HAS BECOME VAPORIZED, AROUND SAID MEAND TEMPERATURE WITHIN A RANGE BETWEEN -1* TO +1*C AND -5* TO +5*C, THEREBY CYCLING THE PRESSURE OF SAID ATMOSPHERE, THE PERIOD OF SAID CYCLING BEING ABOUT 10 TO 30 MINUTES.
 2. A method according to claim 1 in which said compound is gallium arsenide.
 3. A method according to claim 1 in which the single crystal is grown in a horizontal Bridgman type furnace. 